Rotary vane pumps having self-lubricating sliding vanes have been used for several years for a multitude of mechanical and industrial applications and are exposed to a wide range of environmental conditions. These pumps can be used in both gas and liquid pumping applications. One type of rotary sliding vane pump is a dry air pump. In the general aviation field prior to the early 1960's, pumps that were lubricated by oil drove the vacuum systems that powered gyros. These types of pumps were referred to in the art as wet pumps. In the 1960's, the oil lubricated, or wet vane vacuum pumps, were replaced by dry vacuum pumps having carbon vanes and rotors that were self-lubricating. Presently, standard dry vacuum pumps in the market comprise mechanical carbon rotors and vanes operating in a hardened metal ellipsoidal cavity. These pumps provide a power source for, among other things, gyroscopically controlled, pneumatically operated flight instruments.
A dry air type rotary vane pump has a rotor with radially extending slots with respect to the rotor's axis of rotation, vanes that reciprocate within these slots, and a chamber contour within which the vane tips trace their path as they rotate and reciprocate within their rotor slots. The reciprocating vanes thus extend and retract synchronously with the relative rotation of the rotor and the shape of the chamber surface in such a way as to create cascading cells of compression and/or expansion, thereby providing the essential components of a pumping machine.
Certain parts of these pumps can be made of carbon or carbon graphite. These parts rub against other stationary or moving parts of the pump during operation. Graphite from these parts is deposited on the opposing parts by the rubbing action and forms a low friction film between the parts, thereby providing lubrication. The deposited graphite film is itself worn away by continued operation of the pump, and is eventually exhausted out of the pump. The film is replaced by further wear of the carbon graphite parts. Thus, lubrication is provided on a continuous basis that continuously wears away the carbon graphite parts. The pump vanes require and provide the majority of lubrication. Therefore, the vanes wear and lose length as the pump operates. At some point in time, the length of the vanes will become so short that they will not slide properly in the slot, which may lead to pump failure.
Failure of a dry air pump while in service can render one or more aircraft systems inoperative. In addition, most pump failures occur in flight. Dry air pump performance is generally unaffected by wear on the vanes until total failure. Moreover, pump efficiency does not typically degrade enough to be noticed by the pilot until total failure. Usually, pump operation is monitored based on the aircraft's vacuum gauge. If the pump is not operating correctly, the vacuum gauge will indicate such. However, this generally does not occur until near complete failure of the pump.
Previous dry air pump designs typically operate until failure occurs with little deterioration in pumping performance. In other words, the pumping efficiency remains high until actual failure occurs. As a result, indicators in the cockpit indicate, “ok” until the pump fails. Typically, there is no warning in the cockpit that the wear state of the vanes is such that failure can be expected in the reasonably foreseeable future life of the pump. Such a warning is not currently available in the industry.
Occasionally aircraft dry air pumps do wear to the point that performance deteriorates sufficiently to show on a cockpit indicator prior to failure. However, such cases are anomalies. The present state of the art provides lights, gages, etc. in the cockpit to indicate pump failure, after the fact. Except for those rare occasions in which pump wear progresses to such an advanced state prior to failure that pump performance deteriorates, they do not provide information relative to the wear state of the pump or a warning of likely pump failure.
Improved economics for aircraft operations may be achieved through the ability to schedule the replacement of a pump rather than have a pump fail unexpectedly. At present, the only method of reducing the likelihood of unexpected failure is to replace a pump with a serviceable one at an early stage of its life. This “arbitrary” replacement is wasteful.
Characteristically, dry air pump performance is little affected by vane wear until the time of failure, at which time performance collapses totally and instantly. Even though the vanes may have reached a very advanced state of wear prior to failure, efficiency typically does not degrade substantially. What loss of performance that does occur is not typically sufficient to be detected on an aircraft's vacuum gage or other normal cockpit indicators. Thus, the pilot has no warning of an imminent air pump failure.
A correlation exists between the remaining length of the vanes and the expected future operational life of the pump. It has been shown that the incidence of structural failure of the vane/rotor combination begins to increase appreciably after the vanes wear to a certain length. The rate of failure per unit of time increases dramatically as the vanes continue to wear shorter.
When the vane length is equal to approximately 74% or more of its original length, failure due to mechanical malfunction arising from reduced vane length is unlikely. (It may occur in pumps operated at excessive pressure/vacuum, but typically does not occur in normally loaded pumps). The total failure rate (from all causes) for pumps with vanes having remaining lengths greater than 74% is less than approximately 5% of the operating population. Other modes of failure unrelated to vane length might occur at any time during the pump's life.
By the time vane length reaches 68% of original length, about 50% of installed pumps may have failed. More than 90% of those failures are likely to have been caused by mechanical malfunction relating to vane length. By the time vane length falls below 64% of original length, more than 98% of installed pumps may have failed, more than 95% of those failures are related to vane length.
While vane wear occurring as a result of deposition of graphite for lubrication is normal, fairly predictable, and reasonably slow, vane wear is accelerated by operation of carbon graphite parts against roughened interior surfaces of the pump. Such roughness can occur as the result of operating the pump in a harsh environment, with dirty filters, at elevated temperatures or pressures (vacuums), or for a variety of other reasons. Regardless of whether the vanes became worn “normally” at a normal rate, or “abnormally” at an accelerated rate, when the vanes reach the critical length, the likelihood of pump failure increases dramatically. That is to say, regardless of the number of hours of operation, when the vanes wear to a certain length, the likelihood of failure increases dramatically.
Upon rotation of the rotor, the space between each pair of vanes forms a pumping chamber that intakes, compresses, and exhausts air at appropriate points in rotation. For the pump to be efficient, there must be little internal leakage between the individual pumping chambers or the chambers of higher or lower air pressure.
The vanes fit closely in the rotor slots and are fitted closely to the inside of the pumping chamber to prevent the transfer of air from the chamber formed ahead of a vane to the chamber behind the vane. The close fitted vanes prevent transfer of air from or to the exhaust or inlet plenum of the pump, or from the atmosphere, to a chamber of higher or lower pressure. Air leakage from one chamber to the next introduces inefficiency. The pump's output in volume, or pressure (vacuum), or both, deteriorates as a result of the inefficiency.
The nature of the wear and the loading of the parts of the pump normally prevents excess internal leakage, even when the vanes are severely worn away, and even up to the point of imminent failure. However, if “leaks” were introduced between chambers, and those leaks would only occur after the vanes reached a predetermined length, a slow degradation of the pump's performance could be caused, beginning at a predictable time prior to likely failure. The time selected (actually a function of vane length) could be sufficiently early in the pump's life to help insure that the pump was inspected and replaced (if necessary) prior to the vanes reaching an excessive state of wear.
The present invention provides a modification to a rotary pump to introduce deteriorating pumping efficiency as the vane length wears. The deteriorated performance is sufficient and rapid enough to be observed on cockpit indicators, or indicators mounted in other places.